Nearly a third of territory reclaimed from the Islamic State in Iraq and Syria since 2014 has been won in the past six months, due to new policies adopted by the Trump administration, a senior State Department official said Friday.

Brett McGurk, the State Department’s senior envoy to the anti-Islamic State coalition, said that steps President Trump has taken, including delegating decision-making authority down from the White House to commanders in the field, have “dramatically accelerated” gains against the militants.

Combined Islamic State losses in both countries since the group’s peak control in early 2015 total about 27,000 square miles of territory — 78 percent of militant holdings in Iraq and 58 percent in Syria. About 8,000 square miles have been reclaimed under Trump, McGurk said in a briefing for reporters.

He said the Islamic State has been driven out of 45 percent of Raqqa, the group’s de facto Syrian capital since the launch of an offensive by U.S.-backed local forces two months ago. U.S. and coalition airstrikes have been instrumental in the ground successes of the Syrian Democratic Forces, composed of Kurdish and Arab fighters.

(Furat FM)

Assessments by different sources of ground won and lost by the militants over the years have varied widely. Early this year, the defense consultant IHS Jane’s put the total amount of territory controlled by the Islamic State in early 2015 at 35,000 square miles. After “unprecedented” losses for the militants in 2016, it said in January that the group occupied a remaining 23,000 square miles.

Although the Trump administration has yet to announce its new strategy for the campaign against the Islamic State, McGurk cited “key changes” under Trump. In addition to the delegation of decision-making authority, which he said has allowed much greater responsiveness to opportunities and changing circumstances, he cited a “campaign of annihilation” that has concentrated on surrounding cities held by the militants before launching offensives, to ensure that no militants will escape.

He said the 2,000 militants remaining in Raqqa “most likely will die in Raqqa.” The United Nations has estimated that 25,000 civilians also remain in the city.

McGurk, who held the same job in the Obama administration, also cited renewed administration efforts to “increase burden sharing from the coalition” among what he said were 73 countries. Most of them do not contribute to the warfighting but are expected to help with stabilization efforts in cleared areas, including in the Iraqi city of Mosul, where U.S.-backed Iraqi security forces declared victory over the Islamic State last month.

Some U.S. lawmakers have criticized what they have said is a lack of post-conflict plans, while others have said — as Trump emphasized during his presidential campaign — that the United States should not be involved in “nation-building” overseas.

McGurk described extensive preparations, while emphasizing that the United States is not interested in reconstruction or nation-building. Instead, he said, U.S. and partner nations are removing mines, clearing rubble and making sure that basic services — electricity, sewage and water — are operational to allow displaced residents to return under the leadership of local councils.

“People say, ‘We want you to run the hospital, the schools.’ We say, ‘No, we’re not very good at that.’ It’s not our responsibility,” McGurk said.

(Sarah Parnass,Jesse Mesner-Hage/The Washington Post)

Asked whether the current deterioration of relations between the United States and Russia has affected their cooperation in Syria, he said that “so far, we’ve not seen an effect on our engagement” there.

Last month, the two governments announced a cease-fire agreement in southwest Syria, where U.S.-backed opposition forces have battled the Russian-backed Syrian army of President Bashar al-Assad in an ongoing civil war.

Russia, on Assad’s behalf, has also agreed to “deconfliction” lines south of Raqqa and along the Euphrates River, where Assad’s forces threatened to clash with U.S.-backed fighters combating the Islamic State. McGurk described daily “military to military” contacts between Russia and the United States, and less-frequent diplomatic contacts.

Among the successes of the current policy, he said, are the near-cessation of civilian displacements inside Syria and Iraq, and the return of hundreds of thousands of displaced civilians to their homes in places previously occupied by the Islamic State.

Karen DeYoung is associate editor and senior national security correspondent for The Post. In more than three decades at the paper, she has served as bureau chief in Latin America and London and as correspondent covering the White House, U.S. foreign policy and the intelligence community.